Lubricating oils containing complex zinc salts



United States Patent 3,328,335 LUBRICATING OILS CDNTAINING COMPLEX ZINC SALTS Raymond M. .iolie, Cherry Hill, N..l., assignor to Mobil Gil Corporation, a corporation of New York No Drawing. Filed Aug. 31, 1964, Ser. No. 393,402 16 Claims. (Cl. 252--32.7)

This invention relates to a lubricating oil composition. More specifically it relates to lubricating oil compositions having improved stability against oxidation.

It is well known in the art that lubricating oils tend to decompose under the temperatures and pressures of present day engines. The decomposition of the oil is essentially brought about by oxidation to form acids, catalyzed apparently by the very metals present in the engine, iron, copper, and copper alloys, or in the fuels, such as lead. To counteract this deteriorative oxidation of oils, a variety of compounds called antioxidants have been added to lubricating compositions. These agents greatly inhibit the oxygen attack and, thus, lengthen the life of the lubricating oil.

Zinc salts of dialkyldithiophosphoric acids derived from alcohols having 5 or more carbon atoms have been used as antioxidants in engine lubricating oils. These zinc salts are sufiiciently soluble in oil to provide effective antioxidant action.

At present, however, the zinc dithiophosphates prepared by commercial methods and produced entirely from alcohols having less than 5 aliphatic carbon atoms have not been useful as lubricating oil antioxidants because the amounts which can be dissolved are inadequate to prevent oxidation; typical solubilities of these salts therein are less than 1% by weight. This low order of solubility, moreover, substantially excludes the use of these inhibitors as ingredients of additive concentrates which are widely used throughout the petroleum industry in formulating lubricants. As a result, zinc dithiophosphates derived from two of the more economical and widely available alcohols, butyl alcohol and isobutyl alcohol, have, as yet, found little commercial utility as antioxidants.

It is a major object of this invention, therefore, to provide a lubricating oil composition having dissolved therein sufficient amounts of zinc dithiophosphate salt derived from a four-carbon alcohol to provide antioxidant protection. It is also an object of this invention to provide a means of increasing the oil solubility of zinc dithiophosphate salts derived from four-carbon alcohols.

The aforementioned and other objects I achieve by reacting zinc bis(butyl) dithiophosphate or zinc bis(isobutyl) dithiophosphate or zinc oxide complexes thereof with zinc acetate. The exact mechanism of the reaction is not known with certainty. It is believed, however, that in the final product the zinc acetate is coordinated with the zinc oxide complex of the zinc bis(C -alkyl) dithiophosphate, as in those complex salts described in US. Patent No. 3,102,096 to Nygaard et al.

Surprisingly, the solubility of the zinc acetate-coordinated zinc bis(butyl or isobutyl) dithiophosphate salt of this invention in a lubricating mineral oil or a synthetic lubricating oil is so much greater than the untreated salt that stable, non-separating lubricating oil compositions having antioxidant properties may now be formulated therefrom.

In accordance with this invention, by following known commercial procedures, zinc oxide is first reacted with a di(C.,-alkyl) dithiophosphoric acid. The formation of acids of this type is well known, involving a reaction between one mole of phosphorus pentasulfide and four moles of the desired alcohol. Excess zinc oxide, up to about 50% over the stoichiometric requirement, is used. This neutralization reaction takes place in the presence of water 3,328,335 Patented June 27, 1967 at approximately room temperature, in the range of about 20 to 30 C. External cooling is required since the reaction is exothermic. The acid is preferably introduced in an inert, organic solvent, which may afterwards be utilized as an azeotroping agent in removing the water. The final product of this commercial procedure is believed to be a zinc oxide complex of zinc bis(C -alkyl) dithiophosphate.

By using a laborious, non-commercial, multi-step procedure a pure zinc oxide-free zinc salt may be obtained as follows. An ammonium di (C -alkyl) dithiophosphate is first made from the dithiophosphoric acid by neutralization with ammonia. In this reaction, ammonia gas is passed directly into a solution containing the di(C -alkyl) dithiophosphoric acid in an inert organic solvent. Approximately equimolar portions of each reactant are used in forming the ammonium salt. The reaction is also conducted, as in the zinc oxide reaction, at a low temperature, ranging from about 20 to 30 C. Further quantities of solvent may be required to maintain the reaction mixture in a mobile state owing to a thickening of the reaction mass. At the completion of the reaction, the ammonium salt is crystallized from a hot solvent solution.

These crystals are reacted with zinc chloride in an inert solvent-and-water system. The zinc chloride is added in the form of a dilute water solution, using two moles of the ammonium salt per mole of zinc chloride. As in the neutralization step, this reaction is run at a low temperature, in the range of about 20 to 30 C. The zinc chloride is preferably added gradually, even though it is in diluted form. The resulting zinc salt is treated in the same manner as the ammonium salt, by recrystallization from a solvent. The final product of this non-commercial procedure is a simple zinc oxide-free zinc bis(C -alkyl) dithiophosphate.

The zinc acetate complexing reaction is conducted in the presence of water either introduced as hydrated zinc acetate or added to the reaction mixture. The reaction mixture is prepared by first introducing the zinc bis- (c -alkyl) dithiophosphate or the zinc oxide complex thereof in the inert organic solvent. The aqueous zinc acetate, such as zinc acetate dihydrate or preferably a saturated water solution thereof, is then added to the mixture, under agitation at room temperature. After the reactants have been intermixed, they are heated to a temperature in the range of about to 160 C., and preferably about to C. The reaction proceeds at this temperature as the organic solvent and the water are continually being distilled off as an azeot-ropic mixture. The temperature may be allowed to rise above the azeotropic boiling point during the latter stages of the reaction cycle. This procedure of introducing the reactants in different solutions provides a convenient method of mixing the reactants and thereafter removing the water most readily.

The crude reaction product remaining after the water and excess amounts of organic solvent are removed may be cooled and filtered to separate any unreacted zinc acetate or solid by-product.

Analysis indicates that the final acetate-complex product consists of zinc acetate complex both with zinc bis(C -alkyl) dithiophosphate and with zinc oxide-complexed zinc bis=(C -alkyl) dithiophosphate. It is believed that water in the hydrated zinc acetate can convert a portion of the zinc acetate complex to the zinc oxide complex. This hydrolysis may also be caused by the water introduced as a solvent for zinc acetate. Thus, zinc oxidecontaining products are obtained from both the uncomplexed zinc salt prepared by the non-commercial, multistep procedure as well as from the commercial type zinc salt when water is present during zinc acetate complexing.

The zinc acetate complexes of this invention, instead of being insoluble solids, as are the untreated zinc salts,

are viscous liquids. -I have discovered that initial solubility in a typical mineral oil is considerably greater than that of the orginial zinc salt. Furthermore, the complex salt remains dissolved in the oil, while the original acetatefree salt separates out during storage.

The following examples and test results illustrate the typical manner of carrying out and utilizing this invention although there is no intention to limit the invention thereby. Any reference to parts or percentages will be deemed to be on a weight basis.

EXAMPLE I.-COMMERCIAL PREPARATION OF ZINC BIS(ISOBUTYL) DITH-IOPHOSPHATE -Into a four-necked flask, equipped with a stirrer, condenser, additional funnel, and thermometer, were added 296 grams (4.0 moles) of isobutyl alcohol, and heated to 75 C. At that temperature, 222 grams (1.0 mole) of phosphorus pentasulfide were added portionwise, over an hour period, with agitation. After all of the sulfide reactant had been introduced, the temperature was raised to 90 C. and held for 3 hours. The reaction product was cooled and filtered.

The product was added dropwise, with agitation, into a second reactor containing 122 grams (1.5 moles) of zinc oxide, 365 cc. of distilled water, and 200 cc. of benzene, at 60 C. The addition lasted several hours. Thereafter the temperature was raised to distill off the water and benzene as an azeotrope, the final temperature reaching 97 C. The residual mixture was cooled and filtered through Super-Gel (a diatomaceous earth filter-aid) and the remaining benzene was stripped from the filtrate in vacuo. A paraffinic mineral oil was added to dilute the remaining product. The amount of product recovered was 697.7 grams, with 170 grams (24.4% by weight) being oil. This represents a yield of 96.3%.

The product, which becomes a crystalline solid at room temperature, is identified after purification as a zinc oxide complex of zinc bis(isobutyl) dithiophosphate; melting point, 146 C.

Analysis.Found: Percent P=7.95, Percent S=16.0, Percent Zn=l0.4.

EXAMPLE IL-ZINC ACETATE COORDINATION OF COMMERCIALLY PREPARED ZINC BIS(ISO- BUTYL) DITHIOPHOSPHATE Into a reactor similar to Example I, were charged 145 grams of the oil-diluted product of Example 1, containing 109.5 grams (0.2 mole) of the zinc salt, and 335 cc. of benzene. Into this solution was introduced a solution of 48.4 grams (0.22 mole) of zinc acetate dihydrate in 95 cc. of warm distilled water. The mixture was heated to above 80 C. during which water and benzene distilled 01f as an azetrope. The reaction mixture was then cooled and filtered through Super-Cel. Residual benzene was distilled off in vacuo, over a boiling water bath.

The amount of recovered product was 161.7 grams, containing 19.6% of diluent oil, indicating a yield of 88.7%. Infrared spectrum of this product showed the presence of acetate '(carboxylate band at about 6.3 microns).

Analysis-Found: Percent P=7.2-6, Percent S=14.9, Percent Zn=13.4.

Examples 111 and IV describe a non-commercial procedure forpreparing the zinc oxide-free zinc salt.

EXAMPLE III.PREPARATION OF AMMONIUM DIISOBUTYL DITHIOPHOSPHATE Into a dry, 4-necked flask equipped with a stirrer, thermometer, ammonia inlet tube and reflux condenser were added 484 grams (2.0 moles) of diisobutyl dithiophosphoric acid. This material had been previously prepared by the reaction of 4.0 moles of isobutyl alcohol and 1.0 mole of phosphorus pentasulfide as described in Example 1. One liter of cyclohexane was added and the solution was stirred, as 34 grams (2.0 moles) of ammonia gas were introduced. The reaction temperature was in the range of 25 to 31 C. The reaction mixture commenced to thicken into a heavy slurry during the reaction and two 250 cc. portions of cyclohexane were added to thin it. Finally, the solids were filtered off, dissolved in toluene and recrystallized from hot toluene. The colorless, crystalline product was identified as ammonium diisobutyl dithiophosphate with a melting point of 150 to 151 C.

Analysis.--Calcd.: Percent N=5.4, Percent P=12.0, Percent S=24.8. Found: Percent N=5.3, Percent P: 11.3, Percent S=23.6.

EXAMPLE IV.PREPARATION OF ZINC BIS(ISOBUTYL) DITHIOPHOSPHATE Into a 4-necked flask equipped with a stirrer, thermometer, dropping funnel, and air condenser were added 225 grams (0.87 moles) of ammonium diisobutyl dithiophosphate. This solid was dissolved in 500 cc. of cool distilled water, and thereafter, 500 cc. of cyclohexane were added.

To this mixture was added a zinc chloride solution pre-.

pared by dissolving 60 grams of 98.7% zinc chloride (0.435 mole) in 533 cc. of water. The contents of the flask were stirred while the zinc chloride solution was added, the addition lasting for 52 minutes at substantially room temperature. The mixture was stirred for an additional 45 minutes at 25 C., and the entire reaction product was transferred to a separatory funnel.

The aqueous layer was removed from the reaction mixture and the cyclohexane layer was washed with distilled water with four wash portions of 50 cc. each. The Washed cyclohexane layer was filtered through paper and the filtrate heated. over a boiling water bath at 25 mm. Hg to remove the cyclohexane. The solid residue weighed 220 grams, indicating about a 91.7% yield. The product, recrystallized from methyl alcohol, has a melting point of 111 to 112 C.

Analysis.-Calcd.: Percent P=l1.3, percent S=23.4, percent Zn=12.0. Found: Percent P 1l.2, percent 5:: 22.3, percent Zn=l2.0.

EXAMPLE V.-ZINC ACETATE COORDINATION OF ZINC BIS(ISOBUTYL) DITHIOPHOSPI-IATE Into a 4-necked flask equipped with .a stirrer, thermometer, reflux condenser and distillate receiver were added 44.9 grams (0.082 mole) of zinc diisobutyl dithiophosphate prepared in Example IV and 150 cc. of benzene which was sutficient to dissolve the solid salt. To this solution, accompanied by stirring, was added a solution of 71.9 grams (0.328 mole) of zinc acetate dihydrate in cc. distilled water. The stirred mixture was heated to distill off the water and benzene as an. azeotrope. The maximum temperature obtained during the removal of the solvents was 86 C.

The final reaction mixture was filtered to remove ex cess solid zinc acetate. The filtrate was heated over a water bath at 5.5 mm. Hg, distilling off the remaining benzene. Fifty-five grams of a pale yellow, clear viscous liquid was obtained after the removal of the benzene. This liquid was dissolved in normal-heptane. From this solution further amounts of zinc acetate and a solid byproduct crystallized and were removed. The heptane was then removed by distillation. The liquid residue was approximately 81% of the total product. Analysis of this liquid product had the following results:

Analysis.--Found: Percent P=9.8, percent S=20.8, percent Zn: 16.6.

Solubility of zinc acetate-treated product in refined mineral oil The solubility of the zinc acetate coordinated complex salt of Example II was compared with that of the.

and 1.25% by weight. After 3 /2 weeks the complex solid was found to have remained in solution at all concentrations. The zinc oxide salt showed separation of solid additives even at the lowest concentration. The zinc acetate coordinated complex salt had an oil solubility of over 7.6% by weight while the zinc oxide salt had a maximum solubility in the oil of less than 0.98% by weight.

It may thus be seen that the zinc acetate complexing treatment of the commercially prepared zinc bis(C -alkyl) dithiophosphate salt greatly increases the solubility of the salt in mineral oil to provide a highly effective antioxidant concentration. Moreover, the zinc acetate complex salt remains in solution indefinitely so that this solution may be stored without danger of separation.

The complex salt as prepared in Example V and the zinc oxide-free zinc bis(isobutyl) dithiophosphate of Example IV were dissolved in the same lubricating oil as described above. The solubility of the uncomplexed zinc salt was only 2.7% by weight; the solubility of the zinc acetate coordinated complex salt of this invention was 7.9% by weight. This represents about a 3 to 1 improvement in the solubility of the complex salt over that of the noncommercial uncomplexed salt. The multi-step procedure could not be economically used in large-scale production. It is herein illustrated to indicate that even the zinc oxidefree zinc salt is only slightly more soluble than the commercial product when compared to that of the products of this invention.

Oxidative stability test A series of lubrication compositions was prepared containing various amounts of the zinc acetate-coordinated zinc salt of Example II. The composition was heated to 325 F., and dry air at a rate of liters per hour was passed through it in the presence of iron, copper, aluminum and lead. After 24 hours, the neutralization number of each composition is obtained and plotted against the concentration of that composition. From the plot, an additive stability number is determined, by extrapolation, if necessary. This number indicates the percent additive required to keep the neutralization number of the oil below 2.0. The acceptable maximum limit for satisfactory antioxidants is about 100. The product of Example 11 achieved a rating of 65, indicating a highly acceptable performance as an antioxidant.

Concentrations of the complex salt mixture may range from about 1% to about 8% by weight of the lubricating oil composition. However, generally from 2% to 5% will be sufiicient to provide antioxidant properties to the lubricating oil. The products of this invention may be included in straight mineral oil fractions or synthetic lubricants or in oils or synthetic lubricants containing other additives, such as detergents, pour point depressants, viscosity index improvers, and the like, without detracting from the effectiveness of these additives or without suffering loss of antioxidant activity.

It will be understood that the invention, hereinabove described in conjunction with my preferred embodiments, will include all reasonable modifications thereof, as expressed by the following claims.

I claim:

1. An improved lubricating oil composition comprising a major proportion of a lubricating oil having in admixture therewith a minor amount, suflicient to improve the oxidative stability thereof, of a reaction product of a reaction between a zinc bis (C -alkyl) dithiophosphate and zinc acetate said reaction taking place in the presence of water, wherein the reaction mixture is heated to about 80 to 160 C.

2. The lubricating oil composition of claim 1, wherein the zinc acetate is Zinc acetate dihydrate.

3. The lubricating oil composition of claim 1, wherein the zinc bis(C -alkyl) dithiophosphate is zinc bis (n-butyl) dithiophosphate.

4. The lubricating oil composition of claim 1, wherein the zinc bis(C -alkyl) dithiophosphate is zinc bis(isobutyl) dithiophosphate.

5. The lubricating oil composition of claim 1, wherein the zinc bis(C -alkyl) dithitphosphate is the product of a reaction between zinc oxide and di(C -alkyl) dithiophosphoric acid.

6. The lubricating oil composition of claim 1, wherein the minor amount of the said product is from about 1% to about 8% by Weight of the oil composition.

7. The lubricating oil composition of claim 1 wherein the lubricating oil is a mineral oil.

8. A method of improving the oil solubility of a zinc bis(C -alkyl) dithiophosphate salt by reacting the said salt with zinc acetate dihydrate and heating the reaction mixture to to 160 C.

9. The method of claim 8 wherein the reaction with zinc acetate dihydrate takes place in the presence of water.

10. The method of improving the oil solubility of a zinc bis(C -alkyl) dithiophosphate salt comprising the steps of:

(a) dissolving said salt in an inert organic solvent;

(b) mixing with said dissolved salt, a mixture containing 1) zinc acetate dihydrate and (2) water, 311 a (gtemperature in the range of about 20 C. to

(c) maintaining the resulting mixture at said temperature for at least 30 minutes, and

(d) heating the resulting mixture to a temperature in the range of about 80 C. to 160 C. and removing said insert organic solvent and water as an azetropic mixture.

11. The method of claim 10, wherein the resulting mixture is heated to a temperature in the range of about C. to C.

12. The method of claim 10, wherein the zinc bis(C agryl) dithiophosphate is zinc bis(isobutyl) dithiophosp ate.

13. A method of producing a complex salt by reacting a zinc bis(C -alkyl) dithiophosphate salt with zinc acetate in the prescence of water and by heating the reaction mixture to 80 to C.

14. The product resulting from reaction of a zinc bis (C -alkyl) dithiophosphate with zinc acetate dihydrate according to the method of claim 13.

15. The reaction product resulting from reaction of zinc bis(isobutyl) dithiophosphate and zinc acetate dihydrate according to the method of claim 13.

16. The reaction product resulting from the reaction of zinc bis(n-butyl) dithiophosphate and zinc acetate dihydrate according to the method of claim 13.

References Cited UNITED STATES PATENTS 2,790,766 4/1957 Otto 252-32.5 3,102,096 8/1963 Nygaard et al 25232.7 3,290,347 12/1966 Miller 260-4299 FOREIGN PATENTS 550,586 12/1957 Canada.

DANIEL E. WYMAN, Primary Examiner. P. P. GARVIN, Assistant Examiner. 

1. AN IMPROVED LUBRICATING OIL COMPOSITION COMPRISING A MAJOR PROPORTION OF A LUBRICATING OIL HAVING IN ADMIXTURE THEREWITH A MINOR AMOUNT, SUFFICIENT TO IMPROVE THE OXIDATIVE STABILITY THEREOF, OF A REACTION PRODUCT OF A REACTION BETWEEN A ZINC BIS (C4-ALKYL) DITHIOPHOSPHATE AND ZINC ACETATE SAID REACTION TAKING PLACE IN THE PRESENCE OF WATER, WHEREIN THE REACTION MIXTURE IS HEATED TO ABOUT 80* TO 160*C. 